Laser-irradiated thin films having variable thickness

ABSTRACT

A crystalline film includes a first crystalline region having a first film thickness and a first crystalline grain structure; and a second crystalline region having a second film thickness and a second crystalline grain structure. The first film thickness is greater than the second film thickness and the first and second film thicknesses are selected to provide a crystalline region having the degree and orientation of crystallization that is desired for a device component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation application of and claimspriority under 35 U.S.C. §120 to U.S. patent application Ser. No.11/651,305, filed Jan. 9, 2007 and entitled “Laser-Irradiated Thin FilmsHaving Variable Thickness,” the entire contents of which areincorporated herein by reference, which is a divisional application ofand claims priority to U.S. patent application Ser. No. 10/754,157,filed Jan. 9, 2004 and entitled, “Laser-Irradiated Thin Films HavingVariable Thickness,” the entire contents of which are incorporatedherein by reference and which claims priority under 35 U.S.C. §119(e) toU.S. Provisional Patent Application Ser. No. 60/503,424 filed Sep. 16,2003, which is hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to methods and systems for processing thin films,and more particularly to forming crystalline semiconductor thin filmsfrom amorphous or polycrystalline thin films using laser irradiation. Inparticular the present invention relates to a method and system for theproduction of integrated thin film transistors.

BACKGROUND OF THE INVENTION

In recent years, various techniques for crystallizing or improving thecrystallinity of an amorphous or polycrystalline semiconductor film havebeen investigated. This technology is used in the manufacture of avariety of devices, such as image sensors and active-matrixliquid-crystal display (AMLCD) devices. In the latter, a regular arrayof thin-film transistors (TFT) is fabricated on an appropriatetransparent substrate, and each transistor serves as a pixel controller.

Semiconductor films are processed using excimer laser annealing (ELA),in which a region of the film is irradiated by an excimer laser topartially melt the film and then is crystallized. The process typicallyuses a long, narrow beam shape that is continuously advanced over thesubstrate surface, so that the beam can potentially irradiate the entiresemiconductor thin film in a single scan across the surface ELA produceshomogeneous small grained polycrystalline films; however, the methodoften suffers from microstructural non-uniformities which can be causedby pulse to pulse energy density fluctuations and/or non-uniform beamintensity profiles.

Sequential lateral solidification (SLS) using an excimer laser is onemethod that has been used to form high quality polycrystalline filmshaving large and uniform grains. SLS produces large grains and controlsthe location of grain boundaries. A large-grained polycrystalline filmcan exhibit enhanced switching characteristics because the number ofgrain boundaries in the direction of electron flow is reduced. SLSsystems and processes are described in U.S. Pat. Nos. 6,322,625,6,368,945, and 6,555,449 issued to Dr. James Im, and U.S. patentapplication Ser. No. 09/390,537, the entire disclosures of which areincorporated herein by reference, and which are assigned to the commonassignee of the present application.

In an SLS process, an initially amorphous (or small grainpolycrystalline) film is irradiated by a very narrow laser beamlet. Thebeamlet is formed by passing a laser beam through a patterned mask,which is projected onto the surface of the film. The beamlet melts theamorphous film, which then recrystallizes to form one or more crystals.The crystals grow primarily inward from edges of the irradiated area.After an initial beamlet has crystallized a portion of the amorphousfilm, a second beamlet irradiates the film at a location less than thelateral growth length from the previous beamlet. In the newly irradiatedfilm location, crystal grains grow laterally from the crystal seeds ofthe polycrystalline material formed in the previous step. As a result ofthis lateral growth, the crystals attain high quality along thedirection of the advancing beamlet. The elongated crystal grains areseparated by grain boundaries that run approximately parallel to thelong grain axes, which are generally perpendicular to the length of thenarrow beamlet. See FIG. 6 for an example of crystals grown according tothis method.

When polycrystalline material is used to fabricate electronic devices,the total resistance to carrier transport is affected by the combinationof barriers that a carrier has to cross as it travels under theinfluence of a given potential. Due to the additional number of grainboundaries that are crossed when the carrier travels in a directionperpendicular to the long grain axes of the polycrystalline material orwhen a carrier travels across a large number of small grains, thecarrier will experience higher resistance as compared to the carriertraveling parallel to long grain axes. Therefore, the performance ofdevices such as TFTs fabricated on polycrystalline films will dependupon both the crystalline quality and crystalline orientation of the TFTchannel relative to the long grain axes.

Devices that use a polycrystalline thin film often do not require thatthe entire thin film have the same system performance and/or mobilityorientation. For example, the mobility requirements for the TFT columnand row drivers (the integration regions) are considerably greater thanfor the pixel controllers or pixel regions. Processing the entire filmsurface, e.g., the integration regions and the pixel regions, under theconditions necessary to meet the high mobility requirements of theintegration regions can be inefficient and uneconomical since excessirradiation and processing time of the lower performance regions of thethin film may have been expended with no gain in system performance.

SUMMARY OF THE INVENTION

The present invention recognizes that films of different thicknesseshave different film properties. In particular, it is observed that forsimilarly processed films a thicker film exhibits a higher carriermobility than a thinner film. This is observed for all directionalsolidification processes, such as CW-laser scanning, sequential lasersolidification and zone melt refinement, and is true for films that havebeen processed, for example, using an excimer laser, a solid-state laseror a continuous wave laser as the laser source.

The present invention provides a crystalline film containing a firstcrystalline region having a first film thickness that is processed in acrystallization process to provide a first crystalline grain structure.The film further contains a second crystalline region having a secondfilm thickness that is processed in a crystallization process to providea second crystalline grain structure. The first and second filmthicknesses are different and are selected to provide crystallineregions having selected degrees and orientations of crystallization.Typically, the region of greater thickness can contain the longer grainsin the direction of crystal growth. Thicker films also often possesswider grains. The film is suitable for use, for example, in anintegrated circuit device or as an active channel in a thin filmtransistor (TFT). The film may be a semiconductor material or a metal.

In one aspect of the invention, a method for processing a film includes(a) generating a first laser beam pattern from a pulsed laser beam, thelaser beam pattern having an intensity that is sufficient to at leastpartially melt at least a portion of a first region of a film to becrystallized; (b) generating a second laser beam pattern from a pulsedlaser beam, the second laser beam pattern having an intensity that issufficient to at least partially melt at least a portion of a secondregion of the film to be crystallized, wherein the first region of thefilm comprises a first thickness and the second region of the filmcomprises a second thickness, and the first and second thicknesses aredifferent; (c) irradiating the first region of the film with the firstset of patterned beamlets to form a first crystalline region having afirst grain structure; and (d) irradiating the second region of the filmwith the second set of patterned beamlets to form a second crystallineregion having a second grain structure. The laser beam pattern includesa “set” of patterned beamlets, and the set of patterned beamletsincludes one or more laser beamlets.

In one or more embodiments, the method further includes after step (c),repositioning the first laser beam pattern on the film to illuminate asecond portion of the first region of the film, and irradiating thefirst region of the film as in step (c), the steps of repositioning andirradiating occurring at least once; and after step (d), repositioningthe second laser beam pattern on the film to illuminate a second portionof the second region of the film, and irradiating the second region ofthe film as in step (d), the steps of repositioning and irradiatingoccurring at least once.

In one or more embodiments, the irradiation conditions are selected fromthose suitable for sequential laser solidification (SLS), excimer laserannealing (ELA) and uniform grain structure (UGS) crystallization. Aplurality of laser beam sources can be used to generate a plurality oflaser beam patterns. The plurality of laser beam sources can be used toirradiate the same or different regions of the film.

BRIEF DESCRIPTION OF THE DRAWING

Various objects, features, and advantages of the present invention canbe more fully appreciated with reference to the following detaileddescription of the invention when considered in connection with thefollowing drawing, in which like reference numerals identify likeelements. The following drawings are for the purpose of illustrationonly and are not intended to be limiting of the invention, the scope ofwhich is set forth in the claims that follow.

FIG. 1 is a cross-sectional illustration of a crystalline film havingmultiple film thickness regions according to one or more embodiments ofthe present invention.

FIG. 2A illustrates the process of excimer laser annealing according toone or more embodiments of the present invention.

FIG. 2B is an exemplary system for performing sequential lateralsolidification according to one or more embodiments of the presentinvention.

FIG. 3 shows a mask for using in sequential lateral solidificationaccording to one or more embodiments of the present invention

FIG. 4 illustrates a step in the process of sequential lateralsolidification according to one or more embodiments of the presentinvention.

FIG. 5 illustrates a step in the process of sequential lateralsolidification according to one or more embodiments of the presentinvention.

FIG. 6 illustrates a step in the process of sequential lateralsolidification according to one or more embodiments of the presentinvention.

FIG. 7A through FIG. 7C illustrate a sequential lateral solidificationprocess according to one or more embodiments of the present invention.

FIG. 8 is a flow chart of an exemplary process according to one or moreembodiments of the present invention in which two different thicknessregions of the film are processed.

FIG. 9 is an illustration of an apparatus having two optical pathwaysusing a single laser for use in one or more embodiments of the presentinvention.

FIG. 10 is an illustration of an apparatus having two laser systems andtwo optical pathways for use in one or more embodiments of the presentinvention.

FIG. 11 is an illustration of an apparatus having two laser systems,each having two optical pathways, for use in one or more embodiments ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The quality of a film that has been crystallized using a laser-inducedcrystallization growth technique depends, in part, on the thickness ofthe processed film. This observation is used to crystallize differentregions of the film in an energy- and time-efficient manner and toprovide a desired film characteristic. Laser-induced crystallization istypically accomplished by laser irradiation using a wavelength of energythat can be absorbed by the film. The laser source may be anyconventional laser source, including but not limited to, excimer laser,continuous wave laser and solid-state laser. The irradiation beam pulsecan be generated by other known sources for short energy pulses suitablefor melting a semiconductor or metallic material. Such known sources canbe a pulsed solid state laser, a chopped continuous wave laser, a pulsedelectron beam and a pulsed ion beam, and the like.

Films of different thicknesses, although similarly processed, havedifferent film properties. Thick films generally exhibit a higherelectron mobility than similarly processed thin films. “Thick” and“thin” are used here in the relative sense, in that any film that isthicker relative to a second comparative film will exhibit improved filmproperties. A film can be situated on a substrate and can have one ormore intermediate layers there between. The film can have a thicknessbetween 100 Å and 10,000 Å so long as at least certain areas thereof canbe completely or partially melted throughout their entire thickness.While the invention pertains to all films of all thicknesses susceptibleto laser-induced crystallization, “thick” films typically can range fromabout 500 Å (50 nm) to about 10,000 Å (1 μm), and more typically fromabout 500 Å (50 nm) to about 5000 Å (500 nm); and “thin” films typicallycan range from about 100 Å (10 nm) to about 2000 Å (200 nm) and moretypically about 200-500 Å (20-50 nm).

In one or more embodiments, the thin film may be a metal orsemiconductor film. Exemplary metals include aluminum, copper, nickel,and molybdenum. Exemplary semiconductor films include conventionalsemiconductor materials, such as silicon, germanium, andsilicon-germanium. It is also possible to use other elements orsemiconductor materials for the semiconductor thin film. An intermediatelayer situated beneath the semiconductor film can be made of siliconoxide, silicon nitride and/or mixtures of oxide, nitride or othermaterials that are suitable for use as a thermal insulator to protectthe substrate from heat or as a diffusion barrier to prevent diffusionof impurities from the substrate to the film.

Although thick films demonstrate higher mobilities, it is more costlyand time intensive to process them. For example, higher energy densitiesmay be required in order to entirely melt through the thickness of thefilm. Since higher energy density is typically achieved by concentratingthe laser beam into a smaller beam shape (cross-sectional area), smallersections of the film surface can be processed at a time, so that samplethroughput is reduced.

Thus according to one or more embodiments of the present invention, asemiconductor film to be crystallized having regions of differentheights (film thicknesses) is provided. In those regions of the filmswhere high electron mobility is required for optimal device function,the semiconductor film layer is “thick.” In those regions of the filmwhere lower electron mobility is adequate for device performance, a“thin” film is deposited. Thus, thick films are located only in thoseregions of the substrate requiring high speed or mobility, and the thickfilm regions are processed using a slower, more energy intensivecrystallization process. The remaining surface (which is typically thebulk of the surface) is a thin film that is processed more rapidly usinga low cost, low energy crystallization process.

FIG. 1 is a cross-sectional illustration of a thin film article 100having multiple film thicknesses according to one or more embodiments ofthe present invention. A film 110 is deposited on a substrate 120. Thefilm 110 has regions of different film thicknesses. Region 125 of thefilm has a film thickness t₁ that is greater than that of region 130having a thickness of t₂. By way of example, t₁ is in the range of about50-200 nm, and t₂ is in the range of about 20-50 nm. In addition, thepolycrystalline grain structures of regions 125 and 130 differ. Thegrain structure may be polycrystalline or have large single crystallinesubdomains. Region 125 possesses fewer grain boundaries or other defectsper unit area than region 130; and region 125 has a higher mobility.Although the actual mobilities of the regions will vary dependent uponthe composition of the film and the particular lateral crystallizationtechniques used, thick region 125 typically has a mobility in the rangeof greater than about 300 cm²/V-s or about 300-400 cm²/V-s and thinregions 130 typically have a mobility in the range of less than about300 cm²/V-s. In one or more embodiments of the present invention,regions 125 are the active channel regions for a high mobility device,such as a TFT integration region and region 130 is an active channel fora low mobility device such as a pixel control device. In one or moreembodiments, the single crystalline subdomains of the crystallineregions are large enough to accommodate an active channel of anelectronic device such as a TFT.

Improvements in crystal properties typically are observed regardless ofthe specific crystallization process employed. The films can belaterally or transversely crystallized, or the films can crystallizeusing spontaneous nucleation. By “lateral crystal growth” or “lateralcrystallization,” as those terms are used herein, it is meant a growthtechnique in which a region of a film is melted to the film/surfaceinterface and in which recrystallization occurs in a crystallizationfront moving laterally across the substrate surface. By “transversecrystal growth” or “transverse crystallization,” as those terms are usedherein, it is meant a growth technique in which a region of film ispartially melted, e.g., not through its entire thickness, and in whichrecrystallization occurs in a crystallization front moving through thefilm thickness, e.g., from the film surface towards the center of thefilm in a direction transverse to that of the above-described lateralcrystallization. In spontaneous nucleation, crystal growth isstatistically distributed over the melted regions and each nucleus growsuntil it meets other growing crystals. Exemplary crystallizationtechniques include excimer laser anneal (ELA), sequential lateralsolidification (SLS), and uniform grain structure (UGS) crystallization.

Referring to FIG. 2A, the ELA process uses a long and narrow shaped beam150 to irradiate the thin film. In ELA, a line-shaped and homogenizedexcimer laser beam is generated and scanned across the film surface. Forexample, the width 160 of the center portion of the ELA beam can be upto about 1 cm, typically about 0.4 mm, and the length 170 can be up toabout 70 cm, typically about 400 mm, so that the beam can potentiallyirradiate the entire semiconductor thin film 180 in a single pass. Theexcimer laser light is very efficiently absorbed in, for example, anamorphous silicon surface layer without heating the underlyingsubstrate. With the appropriate laser pulse duration (approx. 20-50 ns)and intensity (350-400 mJ/cm²), the amorphous silicon layer is rapidlyheated and melted; however, the energy dose is controlled so that thefilm is not totally melted down to the substrate. As the melt cools,recrystallization into a polycrystalline structure occurs. Line beamexposure is a multishot technique with an overlay of 90% to 99% betweenshots. The properties of silicon films are dependent upon the dosestability and homogeneity of the applied laser light. Line-beam exposuretypically produces films with an electron mobility of 100 to 150 cm²/Vs.

Referring to FIG. 2B, an apparatus 200 is shown that may be used forsequential lateral solidification and/or for uniform grain structurecrystallization. Apparatus 200 has a laser source 220. Laser source 220may include a laser (not shown) along with optics, including mirrors andlenses, which shape a laser beam 240 (shown by dotted lines) and directit toward a substrate 260, which is supported by a stage 270. The laserbeam 240 passes through a mask 280 supported by a mask holder 290. Thelaser beam pulses 240 generated by the beam source 220 provide a beamintensity in the range of 10 mJ/cm² to 1 J/cm², a pulse duration in therange of 10 to 300 ns, and a pulse repetition rate in the range of 10 Hzto 300 Hz. Currently available commercial lasers such as Lambda Steel1000 available from Lambda Physik, Ft. Lauderdale, Fla., can achievethis output. Higher laser energy and larger mask sizes are contemplatedas laser power increases. After passing through the mask 280, the laserbeam 240 passes through projection optics 295 (shown schematically). Theprojection optics 295 reduces the size of the laser beam, andsimultaneously increases the intensity of the optical energy strikingthe substrate 260 at a desired location 265. The demagnification istypically on the order of between 3× and 7× reduction, preferably a 5×reduction, in image size. For a 5× reduction the image of the mask 280striking the surface at the location 265 has 25 times less total areathan the mask, correspondingly increasing the energy density of thelaser beam 240 at the location 265.

The stage 270 is a precision x-y stage that can accurately position thesubstrate 260 under the beam 240. The stage 270 can also be capable ofmotion along the z-axis, enabling it to move up and down to assist infocusing or defocusing the image of the mask 280 produced by the laserbeam 240 at the location 265. In another embodiment of the method of thepresent invention, it is preferable for the stage 270 to also be able torotate.

In uniform grain structure (UGS) crystallization, a film of uniformcrystalline structure is obtained by masking a laser beam so thatnon-uniform edge regions of the laser beam do not irradiate the film.The mask can be relatively large, for example, it can be 1 cm×0.5 cm;however, it should be smaller than the laser beam size, so that edgeirregularities in the laser beam are blocked. The laser beam providessufficient energy to partially or completely melt the irradiated regionsof the thin film. UGS crystallization provides a film having an edgeregion and a central region of uniform fine-grained polycrystals ofdifferent sizes. In the case where the laser irradiation energy is abovethe threshold for complete melting, the edge regions exhibit large,laterally grown crystals. In the case where the laser irradiation energyis below the threshold for complete melting, grain size will rapidlydecrease from the edges of the irradiated region. For further detail,see U.S. application Ser. No. 60/405,084, filed Aug. 19, 2002 andentitled “Process and System for Laser Crystallization Processing ofSemiconductor Film Regions on a Substrate to Minimize Edge Areas, andStructure of Such Semiconductor Film Regions,” which is herebyincorporated by reference.

Sequential lateral solidification is a particularly useful lateralcrystallization technique because it is capable of grain boundarylocation-controlled crystallization and provides crystal grain ofexceptionally large size. Sequential lateral solidification produceslarge grained semiconductor, e.g., silicon, structures throughsmall-scale translations between sequential pulses emitted by an excimerlaser. The invention is described with specific reference to sequentiallateral solidification of an amorphous silicon film; however, it isunderstood that the benefits of present invention can be readilyobtained using other lateral crystallization techniques or other filmmaterials.

FIG. 3 shows a mask 310 having a plurality of slits 320 with slitspacing 340. The mask can be fabricated from, for example, a quartzsubstrate and includes a metallic or dielectric coating that is etchedby conventional techniques to form a mask having features of any shapeor dimension. In one or more embodiments of the present invention, thelength of the mask features is commensurate with the dimensions of thedevice that is to be fabricated on the substrate surface. The width 360of the mask features also may vary. In one or more embodiments of thepresent invention, it is small enough to avoid small grain nucleationwithin the melt zone, yet large enough to maximize lateral crystallinegrowth for each excimer pulse. By way of example only, the mask featurecan have a length of about 25 to about 1000 micrometers (μm) and a widthof about two to five micrometers (μm).

An amorphous silicon thin film sample is processed into a single orpolycrystalline silicon thin film by generating a plurality of excimerlaser pulses of a predetermined fluence, controllably modulating thefluence of the excimer laser pulses, homogenizing the modulated laserpulses, masking portions of the homogenized modulated laser pulses toobtain a laser beam pattern, irradiating an amorphous silicon thin filmsample with the laser beam pattern to effect melting of portions thereofirradiated by the beamlets, and controllably translating the sample withrespect to the laser beam pattern (or vice versa) to thereby process theamorphous silicon thin film sample into a single crystal or grainboundary-controlled polycrystalline silicon thin film. In one or moreembodiments of the sequential lateral solidification process, highlyelongated crystal grains that are separated by grain boundaries that runapproximately parallel to the long grain axes are produced. The methodis illustrated with reference to FIG. 4 through FIG. 6.

FIG. 4 shows the region 440 prior to crystallization. A pulsed laserbeam pattern is directed at the rectangular area 460 causing theamorphous silicon to melt. Crystallization is initiated at solidboundaries of region 460 and continues inward towards centerline 480.The distance the crystal grows, which is also referred to as thecharacteristic lateral growth length, is a function of the amorphoussilicon film thickness and the substrate temperature; however, theactual lateral growth length may be shorter if, for example, the growingcrystals encounter a solid front. A typical lateral growth length for 50nm thick film is approximately 1.2 micrometers. After each pulse thesubstrate (or mask) is displaced by an amount not greater than theactual lateral growth length. In order to improve the quality of theresultant crystals, the sample is advanced much less than the lateralcrystal growth length, e.g., not more than one-half the lateral crystalgrowth length. A subsequent pulse is then directed at the new area. Bydisplacing the image of the slits 460 a small distance, the crystalsproduced in preceding steps act as seed crystals for subsequentcrystallization of adjacent material. By repeating the process ofadvancing the image of the slits and firing short pulses, the crystalgrows epitaxially in the direction of the slits' movement.

FIG. 5 shows the region 440 after several pulses. As is clearly shown,the area 500 that has already been treated has formed elongated crystalsthat have grown in a direction substantially perpendicular to the lengthof the slit. Substantially perpendicular means that a majority of linesformed by crystal grain boundaries 520 could be extended to intersectwith dashed centerline 480.

FIG. 6 shows the region 440 after several additional pulses followingFIG. 5. The crystals have continued to grow in the direction of theslits' movement to form a polycrystalline region. The slits preferablycontinue to advance at substantially equal distances. Each slit advancesuntil it reaches the edge of a polycrystalline region formed by the slitimmediately preceding it.

The many microtranslations called for by the sequential lateralsolidification process increase processing time; however, they produce afilm having highly elongated, low defect grains. In one or moreembodiments, this process is used to process the thick regions of thesemiconductor film. The polycrystalline grains obtained using thisprocess are typically of high mobility, e.g., 300-400 cm²/V-s. This isthe value typically found for devices with having parallel grainboundaries but few perpendicular grain boundaries. These highlyelongated grains are well suited for the active channel regions inintegration TFTs.

According to the above-described method of sequential lateralsolidification, the entire film is crystallized using multiple pulses.This method is hereinafter referred to as an “n-shot” process, alludingto the fact that a variable, or “n”, number of laser pulses (“shots”) isrequired for complete crystallization. Further detail of the n-shotprocess is found in U.S. Pat. No. 6,322,625, entitled “CrystallizationProcessing of Semiconductor Film Regions on a Substrate and Devices MadeTherewith,” and in U.S. Pat. No. 6,368,945, entitled “System forProviding a Continuous Motion Sequential Lateral Solidification,” bothof which are incorporated in their entireties by reference.

In one or more embodiments, regions of the semiconductor film areprocessed using a sequential lateral solidification process thatproduces smaller crystal grains than those of the preceding “n-shot”method. The film regions are therefore of lower electron mobility;however the film is processed rapidly and with a minimum number ofpasses over the film substrate, thereby making it a cost-efficientprocessing technique. These crystallized regions are well suited for thethin film regions of the semiconductor thin film used as the activechannel in pixel control TFTs.

The process uses a mask such as that shown in FIG. 3, where closelypacked mask slits 320 having a width 360, of about by way of example 4μm, are each spaced apart by spacing 340 of about, by way of example, 2μm. The sample is irradiated with a first laser pulse. As shown in FIG.7A, the laser pulse melts regions 710, 711, 712 on the sample, whereeach melt region 720 is approximately 4 μm wide and is spacedapproximately 2 μm apart to provide unmelted region 721. This firstlaser pulse induces crystal growth in the irradiated regions 710, 711,712 starting from melt boundaries 730 and proceeding into the meltregion, so that polycrystalline silicon 740 forms in the irradiatedregions, as shown in FIG. 7B.

The sample is then translated a distance approaching, but more than,half the width of the mask feature, and the film is irradiated with asecond excimer laser pulse. For example, in one embodiment, the sample(or mask) is translated a distance equal to ½ (mask feature width360+mask spacing 340). The second irradiation melts the remainingamorphous regions 742 spanning the recently crystallized regions 740 toform melt regions 751, 752 and 753. The initial crystal seed region 743melts and serves as a site for lateral crystal growth. As shown in FIG.7C, the crystal structure that forms the central section 745 outwardlygrows upon solidification of melted regions 742, so that a uniform, longgrain polycrystalline silicon region is formed.

According to the above-described method of sequential lateralsolidification, the entire mask area is crystallized using only twolaser pulses. This method is hereinafter referred to as a “two-shot”process, alluding to the fact that only two laser pulses (“shots”) arerequired for complete crystallization. Further detail of the two-shotprocess is found in Published International Application No. WO 01/18854,entitled “Methods for Producing Uniform Large-Grained and Grain BoundaryLocation Manipulated Polycrystalline Thin Film Semiconductors UsingSequential Lateral Solidification,” which is incorporated in itsentirety by reference.

According to one or more embodiments of the present invention, a methodfor producing an article having thick film regions of high mobility andthin film regions of low mobility is provided. An exemplary process setforth in the flow diagram 800 of FIG. 8.

In step 810 a thin film having at least two thicknesses is deposited ona substrate, with each film thickness intended to provide crystallineregions having different film properties. In one or more embodiments,the film property of interest is mobility; however other film propertiessuch as crystal orientation, crystal size, and grain defects can also beconsidered. Of course, the film can have more than two film thicknessregions to thereby provide more than two different film properties. Thesize and placement of devices on the film is selected to correspond tothe different film thickness regions. For example, pixel control devicesare located in regions of thinner film thickness and integration devicesare located in regions of thicker film thickness.

The fabrication of films of different thicknesses is known in the art.For example, a film can be deposited evenly across the substrate, andthereafter sections thereof are removed, e.g., etched or polished, toform regions of thicker and thinner film thicknesses. In some exemplaryembodiments, the film is etched back to expose the underlying substrate,and a second layer of semiconductor material is deposited over theexposed substrate and existing semiconductor layer to form a film ofdifferent thicknesses. Alternatively, the film is etched so as to removesome, but not all, of the semiconductor material in the thin filmregions. In other exemplary embodiments, photolithography can be used topattern the film surface, followed by selective deposition or materialremoval in the exposed regions of the patterned substrate.

In step 820 the laser beam conditions (beam shape, beam energy density,beam homogeneity, etc.) and mask design (where present) are selected forprocessing the thick regions of the semiconductor film. The order ofirradiation is not critical to the invention and either thick or thinfilm regions can be processed first, or they can be processedsimultaneously. As is discussed in greater detail below, one or morelaser beam sources may be used to generate the laser beam pattern thatirradiates the film surface. A laser beam generated from a laser beamsource may be split or steered to generate secondary laser beams, eachof which can be shaped using masks and/or laser optics to providepatterned beamlets with desired characteristics.

In step 830 the “thick” film region of the semiconductor film isirradiated to obtain a first crystalline region. According to one ormore embodiments of the present invention, the region is irradiated in asequential lateral solidification “n”-shot process. The firstcrystalline region may include the entire “thick” film region, such thatthe film is crystallized up to the edge of the thick film region. Edgemelting may result in material flow at the interface between the thickand thin films; however, rapid recrystallization and surface tension areexpected to limit material flow. Alternatively, the entire thick filmregion may not be irradiated, forming for example an amorphous borderbetween the “thick” and “thin” film regions.

In step 835, it is determined whether thick film processing is complete.If not, the process returns to step 830 to process a new portion of thethick film region. If thick film region is crystallized, the step iscomplete, and the process advances to the next step.

In step 840 the laser beam conditions (beam shape, beam energy density,beam homogeneity, demagnification, etc.) and mask design are selectedfor processing the thin regions of the semiconductor film. The order ofirradiation is not critical to the invention, and this step is carriedout before, during or after processing of the thick film. As is the casefor the thick film region, one or more laser sources can be used togenerate the laser beamlets used to irradiate the thin film regions ofthe film. In addition, the laser beam generated from a laser beam sourcemay be split or steered to generate secondary laser beams, each of whichcan be shaped using masks and/or laser optics to provide patternedbeamlets with desired characteristics.

In step 850 the “thin” film region of the semiconductor film isirradiated to obtain a second crystalline region. According to one ormore embodiments of the present invention, the region is irradiated in asequential lateral solidification two-shot process. ELA and UGScrystallization can also be used to provide a crystalline region ofuniform grain structure.

In step 855, it is determined whether the thin film processing iscomplete. If not, then the process returns to step 850 and a new portionof the thin film is irradiated. If complete, the process advances tostep 860 and is done.

Variations of the process are contemplated within the scope of thepresent invention. For example, the crystallization method used for thefirst and second regions of the film can be the same or different. Inone or more embodiments, the thick film regions requiring highermobility can be processed using a technique such as SLS that produceselongated, grain boundary location-controlled grain structure, and thethin film regions can be processed using a less expensive technique,such as UGS crystallization. In one or more embodiments of the presentinvention, a portion of the “thick” and/or “thin” regions are processed.The remaining unprocessed portions remain in the as-depositedcrystalline state, e.g., amorphous or small-grained polycrystalline. Thesize and location of the processed and unprocessed regions of the“thick” and/or “thin” regions may be selected, for example, tocorrespond to devices to be located on the film.

By way of further example, even when using the same crystallizationtechnique, the masks for the first and second irradiations can be thesame or different. When the masks are the same, then the conditions ofirradiation typically may vary, as for example described above where an“n”-step and a two-step process are used for the two film thicknessregions. In some embodiments, different masks are used for the first andsecond irradiations. For example, the orientation of the mask featurescan vary so that crystal growth proceeds in different directions on thefilm. Mask orientation can be varied by rotating the mask or thesubstrate stage on which the sample rests or by using different masks.

In some embodiments, the laser features, e.g., the laser beam shape andenergy density, can be modified so that each region of the amorphousfilm is irradiated with a laser beam (i.e., a laser beam pattern) havingdifferent beam characteristics, e.g., beam energy profile (density),beam shape, beam pulse duration, etc. The beam characteristics of thelaser beams being delivered to the amorphous film can be controlled andmodulated via the optical elements, e.g., lenses, homogenizers,attenuators, and demagnification optics, etc., and the configuration andorientation of a mask(s), if present. By modulating the beamcharacteristics of the laser beams in accordance with the processingrequirements (to facilitate crystallization) of the film portion to beirradiated, the laser source's output energy can be more efficientlyutilized in the crystallization fabrication process, which in turn canlead to improved (i.e., shorter) film processing times and/or lowerenergy processing requirements. Accordingly, the laser beams can becontrolled and modulated so that different regions of the film that havedifferent processing requirements are irradiated by laser beams havingdifferent beam characteristics. For example, the “thin” portions of theamorphous film layer can be subjected to laser beams that have certainenergy beam characteristics while the “thick” portions of the film layercan be subjected to laser beams that have different energy beamcharacteristics.

Laser beams having differing energy beam characteristics can begenerated and delivered to the amorphous film using systems that have asingle optical path or, alternatively, have a plurality of opticalpaths. An optical path, as that term is used herein, refers to thetrajectory of a laser beam pulse as the laser beam pulse travels from alaser beam source to a thin film sample. Optical paths thus extendthrough both the illumination and projection portions of the exemplarysystems. Each optical path has at least one optical element that iscapable of manipulating the energy beam characteristics of a laser beampulse that is directed along that optical path.

In systems having a single optical path, one or more of the opticalelements and the mask (if present) can be adjusted, inserted orsubstituted, etc., within the optical path so as provide laser beamletshaving different energy beam characteristics. Additionally, theorientation of the substrate, relative to the orientation of theincoming laser beams, can also be adjusted to effectively produce alaser beam that has different energy beam characteristics. In one ormore embodiments, for example, the laser system can include a mask thatis rotatable via a mask holder. The mask is held in a first position tofacilitate the irradiation processing of a first portion of the film andthen is rotated to a second position, e.g., rotate 90°, to facilitatethe irradiation processing of a second portion of the film. In one ormore embodiments, the laser system can include two masks havingdifferent masking shapes being located on a mask holder. To irradiate afirst portion of the silicon film, the first mask is aligned with thelaser beam optical path via the mask holder. To irradiate a secondportion, the second mask is then aligned with the laser beam opticalpath via the mask holder, e.g., the mask holder can be a rotatable diskcartridge. In yet another embodiment, for example, the system caninclude an adjustable demagnification optical element. To generate laserbeams having differing energy beam characteristics, the adjustabledemagnification optical element is set to a first magnification duringthe irradiation of a portion of the amorphous film and then set to adifferent magnification during the irradiation of another portion of theamorphous film. Thus, laser beams having different energy beamcharacteristics can be generated and delivered to the amorphous film onthe same optical path. Other modification to modify beam characteristicsof a laser beam in a single optical path will be apparent to those ofskill in the art.

Generating laser beam with different beam characteristics along a singleoptical path may cause the crystallization processing times to otherwiseincrease in some circumstances since the delivery of the irradiationenergy to the amorphous film may need to be interrupted to facilitatethe modulation of the energy beam characteristics. In this instance, asystem having a single laser beam path may not be advantageous since thechanging of the optical elements, the mask configuration or orientation,or the substrate orientation, etc., to facilitate an adjustment of thelaser beam characteristics could dramatically lower the duty cycle ofthe delivered laser energy. In one or more embodiments and to generatelaser beams having differing energy beam characteristics whilemaintaining an acceptable delivered irradiation duty cycle, the systemsfor irradiating the amorphous film can include a plurality of opticalpaths. As shown schematically in FIG. 9, in some embodiments the systemcan include two optical paths for controlling and modulating the laserbeam, each of which can include the necessary beam optics, e.g., beamhomogenizers, demagnification optics, mirrors, lenses, etc., and(optionally) a mask to modulate the beam characteristics of the laserbeam and direct the laser beam to portions of the amorphous film so thatcrystallization can be promoted. Accordingly, the dual (or multiple)optical path system can be used to generate laser beams of differentbeam characteristics, which are used to irradiate and crystallize thedifferent film regions of the film. Thus, a first laser beam having afirst set of beam characteristics is generated and delivered via a firstoptical path. A selected portion of the film is irradiated with thefirst laser beam using a first crystallization process to obtain a firstcrystalline region. The crystalline region corresponds to a region ofthe film having a selected film thickness. Upon completion of orconcurrent with the first irradiation step, the laser beam is redirectedonto a second optical path that generates a laser beam having a secondset of beam characteristics. A selected portion of the film isirradiated with the second laser beam using a second crystallizationprocess to obtain a second crystalline region. The crystalline regionscan correspond to regions of the film having different selected filmthicknesses. The crystalline regions can be polycrystalline or havelarge single crystalline domains.

An exemplary apparatus having dual optical paths that can generate anddeliver laser beams having different energy beam characteristics to afilm is shown in FIG. 9. Referring to FIG. 9, the system 900 includes alaser source 220, an attenuator 910, a telescope 920, a homogenizer 930,a condenser lens 940 and a beam steering element 950. The laser beam 240generated by the laser source 220 is directed to the beam steeringelement 950 via the attenuator 910, telescope 920, homogenizer 930 andcondenser lens 940. The attenuator 910, which may be used in conjunctionwith a pulse duration extender, can be a variable attenuator, e.g.,having a dynamic range capable of adjusting the energy density of thegenerated laser beams 240. The telescope 920 can be used to efficientlyadapt the beam profile of the laser beams 240 to the aperture of thehomogenizer 930. The homogenizer 930 can consist of two pairs of lensarrays (two lens arrays for each beam axis) that are capable ofgenerating a laser beam 240 that have a uniform energy density profile.The condenser lens 940 can condense the laser beam 240 onto a downstreamoptical element.

At the beam steering element 950, the incoming laser beams 240 aredirected along one of two different out-going optical paths, each ofwhich leads to the substrate 260 that is mounted on the wafer-handlingstage 270. The first optical path includes a mirror 960, avariable-focus field lens 970 a, a mask 280 a and a projection lens 295a, while the second optical path includes a variable-focus field lens970 b, a mask 280 b and a projection lens 295 b. The masks 280 a and 280b are typically mounted to mask stages (not shown) that are capable ofaccurately positioning the masks (e.g., in three dimensions) inrelationship to the incoming laser beams 240. Laser beams 240 travelingalong the two different optical paths pass through optical elements thathave different optical properties. For example, in one embodiment, themask 280 a of the first optical path has a different maskingconfiguration than the mask 280 b of the second optical path. Thus,laser beam 240 a, which is directed to region 265 a of substrate 260 viathe first optical path, will have energy beam characteristics that aredifferent from the energy beam characteristics of the laser beam 240 bthat is directed to region 265 b of substrate 260 via the second opticalpath. Regions 265 a, 265 b are shown as having different filmthicknesses.

In certain embodiments, the beam steering element 950 can have twomodes: a transmissive or pass-through mode and a reflective or redirectmode. While operating in a pass-through mode, the laser beams 240entering the beam steering element 950 essentially pass completelythrough the beam steering element 950 onto a first optical path. Whileoperating in a redirect mode, the laser beams 240 entering the beamsteering element 950 are essentially completely redirected by areflective surface(s) onto a second optical path.

The wafer-handling stage 270 is capable of accurately positioning thesubstrate 260 masks (e.g., in three dimensions) in relationship to theincoming laser beams 240 a and 240 b. As previously discussed, thedual-thickness amorphous film is deposited in a controlled manner upon asurface of the substrate 260. Laser beam(s) 240 can then be directed tothe second optical path via the beam steering element 950 so that laserbeam(s) 240 b having second beam characteristics are generated anddirected to different portions of the film. Therefore, by coordinating(i.e., via a computer) the generation of the laser beam 240, theoperations of the beam steering element 950 and the positioning of thesubstrate 260 via the wafer-handling stage 270, the delivery of laserbeams 240 a and 240 b (having different beam characteristics) todifferent portions of the amorphous film can be facilitated.

In other exemplary embodiments, the beam steering element 950 is a beamsplitter that allows a portion of the laser beam to pass through thebeam splitter to pathway 240 a and a portion of the laser beam to beredirected along pathway 240 b so that different portions of the thinfilm can be irradiated at the same time.

In other exemplary embodiments, a plurality of laser sources and aplurality of optical paths, such as those described in detail above, canbe employed. Each laser source generates a laser beam(s) that can bedirected along a corresponding optical path so as to produce a laserbeam(s) having specific beam characteristics. The laser beam(s) can thenbe directed via the optical path to a region of the thin film. Forexample, a laser beam(s) from the laser source can be directed along thefirst optical path so that a laser beam(s) having first beamcharacteristics is produced and delivered to certain portions of thefilm while a laser beam(s) from a second laser source can be directedalong a second optical path so that a laser beam(s) having differentbeam characteristics is produced and delivered to certain other portionsof the film. This is illustrated schematically in FIG. 10, in which twolaser sources are shown as boxes 1010 a and 1010 b. The optical elementsof optical pathways 1030 a and 1030 b may be variously arranged as isunderstood in the art and may include some or all of the opticalelements, e.g., beam homogenizers, demagnification optics, mirrors,lenses, etc., that are described herein. Laser beam(s) generated bylaser source 1010 a travel along optical pathway 1030 a (therebyproducing laser beam(s) having certain energy beam characteristics) andare delivered to the “thin” region 1020 a of the thin film. Laserbeam(s) generated by laser source 1010 b travel along optical pathway1030 b (thereby producing laser beam(s) having certain energy beamcharacteristics) and are delivered to the “thick” region 1020 b of thethin film. In certain embodiments, the energy beam characteristics ofthe laser beam(s) that is delivered to the “thin” region 1020 a differsfrom the energy beam characteristics of the laser beam(s) that isdelivered to the “thick” region 1020 b. In certain embodiments of thesystem depicted in FIG. 10, the processing of the “thin” region 1020 aof the thin film is processed either before or after the processing ofthe “thick” region 1020 b of the thin film. In certain other embodimentsof the system depicted in FIG. 10, however, the processing of the “thin”region 1020 a of the thin film is performed concurrently with theprocessing of the “thick” region 1020 b of the thin film.

In some embodiments, a plurality of laser systems, which each use aplurality of optical pathways, can be employed. In such embodiments,each laser system can be made up of one or more laser sources. In suchembodiments, different laser systems can be used to process differentregions of the thin film. For example, laser beams generated by thelaser source(s) of a first laser system and by the laser source(s) of asecond laser system can be directed along two other different opticalpaths so as to process a “thick” region of the thin film. The laserbeam(s) generated by the laser source(s) of the first laser system canbe directed to the corresponding optical paths via a beam steerer or abeam splitter depending upon whether the generated laser beam(s) are tobe split or not. The laser beams(s) of the second laser system can beprocessed and handled similarly. The laser beams that are directed tothe “thin” region may have similar or different energy beamcharacteristics. Similarly, the laser beams that are directed to the“thick” region may have similar or different energy beamcharacteristics. An exemplary embodiment having two independent lasersystems 1210 a and 1210 b with corresponding beam splitters 1230 a and1230 b is depicted in FIG. 11. The laser beams 1220 a and 1220 bgenerated by laser systems 1210 a and 1210 b pass through beam splitters1230 a and 1230 b, respectively. Beam splitter 1230 a directs a portionof laser beam 1220 a onto optical path 1240 a and directs the remainingportion of laser beam 1220 a onto optical path 1240 b so that bothenergy beams (which may have similar or different energy beamcharacteristics) can simultaneously irradiate different portions of the“thin” region 1250 of the thin film. Similarly, beam splitter 1230 bdirects a portion of laser beam 1220 b onto optical path 1260 a anddirects the remaining portion of laser beam 1220 b onto optical path1260 b so that both energy beams (which may have similar or differentenergy beam characteristics) can simultaneously irradiate differentportions of the “thick” region 1280 of the thin film.

In still other embodiments as described above, beam splitters canoperate as a beam steering elements that can operate in a transmissiveor pass-through mode and a reflective or redirect mode. While operatingin a pass-through mode, the laser beams entering the beam steeringelement essentially pass completely through the beam steering elementonto a first optical path. While operating in a redirect mode, the laserbeams entering the beam steering element are essentially completelyredirected by a reflective surface(s) onto a second optical path.

Further detail is provided in co-pending patent application entitled“Systems And Methods For Inducing Crystallization of Thin Films UsingMultiple Optical Paths” filed on even date herewith, and in co-pendingpatent application entitled “Systems And Methods For Processing ThinFilms” filed on even date, the contents of which are incorporated byreference.

The devices fabricated by the present invention include not only anelement such as a TFT or a MOS transistor, but also a liquid crystaldisplay device (TFT-LCDs), an EL (Electro Luminescence) display device,an EC (Electro Chromic) display device, active-matrix organic lightemitting diodes (OLEDs), static random access memory (SRAM),three-dimensional integrated circuits (3-D ICs), sensors, printers, andlight valves, or the like, each including a semiconductor circuit(microprocessor, signal processing circuit, high frequency circuit,etc.) constituted by insulated gate transistors.

Although various embodiments that incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatincorporate these teachings.

1. A device comprising: a plurality of thin film transistors (TFT), eachTFT having a thin film active channel; wherein at least a first TFTactive channel has a first thickness and at least a second TFT activechannel has a second thickness that is greater than the first thicknessand the mobility of the second active channel is greater than themobility of the first active channel.
 2. A device of claim 1, whereinthe first TFT active channel comprises an active channel for anintegration area TFT.
 3. A device of claim 1, wherein the second TFTactive channel comprises an active channel for a pixel area TFT.
 4. Adevice of claim 1, wherein the film comprises a semiconductor material.5. A device of claim 1, wherein the first and/or second crystallineregions comprise a single crystal subregion having a dimension largeenough to accommodate an active channel of a TFT.
 6. A system forprocessing a film comprising: a laser source capable of producing apulsed laser beam; one or more optical elements capable of generating afirst laser beam and a second laser beam having different beamcharacteristics from the pulsed laser beam, wherein the first laser beamhas a first energy density to melt a first region of a film to becrystallized to form a first crystalline region having a first grainstructure and the second laser beam has a second energy densitysufficient to melt a second region of the film to be crystallized toform a second crystallized region having a second grain structure,wherein the first energy density is greater than the second energydensity; a stage configured to support and position a film to becrystallized such that the first laser beam is directed to andirradiates a first portion of the stage and a second laser beam isdirected to and irradiates a second portion of the stage; and a computerfor controlling the laser source, optical elements, and stage tofacilitate the delivery of the first laser beam and the second laserbeam to portions of the stage corresponding to first and second regionsof a film to be crystallized.
 7. The system of claim 6, comprising afilm positioned on the stage.
 8. The system of claim 7, wherein thefirst laser beam irradiates a first region of the film and the secondlaser beam irradiates a second region of the film.
 9. The system ofclaim 8, wherein the irradiations of the first region of the film andthe second region of the film occur substantially at the same time. 10.The system of claim 8, wherein the irradiations of the first region ofthe film and the second region of the film occur sequentially.
 11. Thesystem of claim 7, wherein the first region has a first thickness andthe second region has a second thickness.
 12. The system of claim 11,wherein the first thickness and the second thickness are different. 13.The system of claim 11, wherein the first thickness is greater than thesecond thickness.
 14. The system of claim 7, wherein the film comprisesa semiconductor material.
 15. The system of claim 6 further comprising afirst mask for masking the first laser beam and a second mask formasking the second laser beam.
 16. The system of claim 15, wherein thefirst mask and the second mask have the same masking configuration. 17.The system of claim 15, wherein the first mask and the second mask havedifferent masking configurations.
 18. The system of claim 6, wherein theoptical element comprises a beam steering element to receive the pulsedlaser beam and direct the pulsed laser beam along a first optical pathgenerating the first laser beam and along a second optical pathgenerating the second laser beam.
 19. The system of claim 18, whereinthe beam steering element comprises a beam splitter.
 20. The system ofclaim 18, wherein the beam steering element comprises a mirror.